1. Field of the Invention
The present invention generally relates to a pneumatic tire. The present invention specifically relates to a pneumatic tire capable of suppressing stone trapping.
2. Description of the Related Art
Stones are sometimes trapped within grooves that are formed on the tread area of pneumatic tires of vehicles. When the stones are trapped within the grooves, so-called “stone drilling” may occur. The stone drilling is a phenomenon that stones penetrate the bottoms of the grooves due to rolling of the pneumatic tire to cause damage to the tread area. To take care of this issue, some of the conventional pneumatic tires have protrusions in the grooves to minimize stone trapping in the grooves. Due to the provision of the protrusions, even if stones enter the grooves, the stones are ejected to the outside of the grooves by the elastic force of the protrusion.
When manufacturing pneumatic tires having protrusions in the grooves, however, the protrusions become obstacle to flow of rubber for forming the tread area inward in the tire radial direction of the protrusion. This may increase the pressure of the rubber located inward of the protrusion in the tire radial direction, and associated with this, a breaker ply located inward of the protrusion in the tire radial direction can get deformed into a wavy shape. If the breaker ply is deformed in this manner, abnormal wear may occur to the pneumatic tire due to the deformation in the breaker ply when a vehicle to which the pneumatic tires are fit travels.
Some of the conventional pneumatic tires have a configuration that makes it possible to suppress the deformation of the breaker ply when the protrusions are provided in the grooves. For example, in Japanese Patent Application Laid-Open No. S61-291203, a plurality of protrusions are provided in grooves that extend in a zigzag shape in the tire circumferential direction, and connection members for connecting the protrusions to the sidewalls of the grooves are provided in locations where the adjacent protrusions in the tire circumferential direction are provided alternately in the tire width direction. In such a structure, the rubber located inward of the protrusion in the tire radial direction can escape in the direction of a land during manufacture of the pneumatic tire. It is, therefore, possible to prevent the pressure of the rubber located inward thereof in the tire radial direction from becoming too high. Consequently, it is possible to suppress the deformation of the breaker ply located inward of the protrusion in the tire radial direction and to reduce the abnormal wear.
The protrusion provided in the groove ejects the stone entering the groove to the outside of the groove by the elastic force of the protrusion, and prevents the stone trapped within the groove from reaching the breaker ply by the volume of the protrusion. Therefore, the protrusion needs to have a predetermined height and a predetermined volume to fulfill these functions. Greater effect of suppressing stone trapping can be obtained if the height is larger or if the volume is larger. However, if the protrusion is too large, then the rubber does not satisfactorily flow into a mold for forming the protrusion, and it is difficult to discharge the air present between the mold and the rubber during manufacture of the pneumatic tire. As a result, the pneumatic tire is manufactured without obtaining a targeted shape of the protrusion, which causes failure in manufacture, i.e., occurrence of “bare” (depressed area).